From: Jonathan Nieder Date: Sat, 04 Jan 2020 00:13:31 GMT Subject: Re: [PATCH v2] sha1-file: remove OBJECT_INFO_SKIP_CACHED Message-ID: <20200104001331.GA130883@google.com> In-Reply-To: <20200102201630.180969-1-jonathantanmy@google.com> Jonathan Tan wrote: > In a partial clone, if a user provides the hash of the empty tree ("git > mktree requires that that object be parsed, for example: > > git diff-tree 4b825d > > then Git will lazily fetch the empty tree, unnecessarily, because > parsing of that object invokes repo_has_object_file(), which does not > special-case the empty tree. > > Instead, teach repo_has_object_file() to consult find_cached_object() > (which handles the empty tree), thus bringing it in line with the rest > of the object-store-accessing functions. A cost is Lovely, thank you. > Signed-off-by: Jonathan Tan > --- > object-store.h | 2 -- > sha1-file.c | 38 ++++++++++++++++++-------------------- > 2 files changed, 18 insertions(+), 22 deletions(-) Reviewed-by: Jonathan Nieder To follow up on Junio's hint in his review: callers can inject additional cached objects by using pretend_object_file. Junio described how this would make sense as a mechanism for building the virtual ancestor object, but we don't do that. In fact, the only caller is fake_working_tree_commit in "git blame", a read-only code path. *phew* -- >8 -- Subject: sha1-file: document how to use pretend_object_file Like in-memory alternates, pretend_object_file contains a trap for the unwary: careless callers can use it to create references to an object that does not exist in the on-disk object store. Add a comment documenting how to use the function without risking such problems. The only current caller is blame, which uses pretend_object_file to create an in-memory commit representing the working tree state. Noticed during a discussion of how to safely use this function in operations like "git merge" which, unlike blame, are not read-only. Inspired-by: Junio C Hamano Signed-off-by: Jonathan Nieder --- object-store.h | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/object-store.h b/object-store.h index 55ee639350..d0fc7b091b 100644 --- a/object-store.h +++ b/object-store.h @@ -208,6 +208,14 @@ int hash_object_file_literally(const void *buf, unsigned long len, const char *type, struct object_id *oid, unsigned flags); +/* + * Add an object file to the in-memory object store, without writing it + * to disk. + * + * Callers are responsible for calling write_object_file to record the + * object in persistent storage before writing any other new objects + * that reference it. + */ int pretend_object_file(void *, unsigned long, enum object_type, struct object_id *oid); -- 2.24.1.735.g03f4e72817